Across the United States, some 58,000 wildfires have burned more than 9.2 million acres this year, making the air in many towns and cities too dangerous to breathe. All told, 2017 is now second only to 2015 as the worst wildfire season on record.
In California — where more than 1 million acres and 10,000 structures have burned and 42 people have been killed from fires — the 2017 season is without precedent in its destruction. (And it’s far from over — as of Tuesday morning, the Thomas Fire in Ventura and Santa Barbara counties, which has already burned an area larger than New York City, was only 20 percent contained.)
So what’s going on here? This must be climate change at work, right?
Yes and no.
Scientists have found that human-caused climate change is increasing the frequency and size of wildfires for much of the United States, particularly in forested areas like those that burned in Montana in September and in Northern California in October.
And as this list shows, six of the 10 largest California wildfires occurred in the past decade.
But as with the 2017 hurricane season, it’s hard to tease out the influence of climate change in any individual fire.
And in the case of the Southern California fires, the signal is weaker.
Massive December fires are unusual around Los Angeles. But when it comes to humanity’s role in the destructive blazes, scientists say our habit of building in harm’s way may be a bigger factor in the fire’s devastation than rising temperatures due to burning fossil fuels.
Wildfires do occur on their own in many parts of the United States, but humans now ignite the vast majority of them. As more people crowd into neighborhoods next to dry brush and parched forests, the likelihood of an ember from a barbecue grill or a spark from a lawnmower triggering an inferno shoots up.
And throughout California, property values are rising and populations are growing, increasing the damage totals and the number of people at risk from fires.
This isn’t just an academic discussion. Figuring out what’s driving wildfires is important for identifying where best to allocate resources to protect lives and property while anticipating future risks. (The October wildfires in Northern California already have an estimated price tag of $65 billion.)
California Gov. Jerry Brown described the ongoing blazes as “the new normal.”
“This could be something that happens every year or every few years,” he said. “We’re about to have a firefighting Christmas.”
The unique combination of weather behind the flames in Southern California
For the blazes scorching the southern part of the state right now, there are several key weather elements driving the flames forward.
The moisture accelerated the proliferation of fast-growing grasses and shrubs that dominate the chaparral landscape around Los Angeles. Then a hot summer capped with a record-breaking October heat wave dried out the vegetation.
Finally, season-ending rain hasn’t shown up yet, stalled in part due to cooler Pacific Ocean temperatures under cyclic La Niña conditions.
That left vast swaths of Southern California covered in dry tinder. Then the hot, arid Santa Ana winds picked up with unusual speed, increasing the likelihood of a fire starting and speeding the spread of flames.
The heat, the vegetation, the rain (or lack thereof), and the wind are all parts of natural variations in the region that coincidentally aligned this year, and they haven’t shown any trend consistent with long-term global warming.
“We have not seen substantial changes in autumn burned area in southern California tied to Santa Ana wind events over the past 70 years,” said John Abatzoglou, a climate researcher at the University of Idaho, in an email.
Climate change had a stronger signal in the Northern California fires
This is in contrast to the fires in Northern California, where the climate is temperate year-round. A hotter and drier climate trend leaves a bigger mark on the cooler, wetter San Francisco Bay Area compared to Southern California, which is already hot and dry.
As a result, scientists have found human-caused climate change to be a major contributor to forest fires in the western United States, like the ones that burned through California’s wine country in October.
“In the Sierra Nevada, about 50 percent of the variation [in wildfires] is explained by climate,” said Jon Keeley, a senior scientist at the US Geological Survey and an adjunct professor at the University of California Los Angeles. “When we look at Southern California and coastal California, there’s no relationship between climate change and fire.”
Droughts exacerbated by climate change also have contradictory effects between San Francisco and Los Angeles. Keeley noted that droughts dry out trees, making them more prone to ignite, but they kill off Southern California’s quick-growing bushes and grasses, which actually reduces the fuel available to burn.
State officials on Monday reported a record 129 million dead trees across 8.9 million acres due to drought and bark beetles, posing the biggest wildfire hazard to the Sierra Nevada region in the northern part of the state.
Worse fires are probably in store for Southern California because of climate change
What’s striking about the fires in California this year is that the state does not typically have bad fires in the north and the south during the same season, as it’s had in 2017, which is why the total burned area is unusually large.
This is not to say that climate change doesn’t have any role in Southern California’s blazes, or that a signal will not emerge at some point in the future.
“These December fires are terribly tough to model since we haven’t seen this much amount of fire on the landscape in southern California in the last century,” Abatzoglou said.
In fact, some researchers project that fires driven by Santa Ana winds, and the fires that occur earlier in the year in Southern California, will burn larger areas by midcentury in part due to rising temperatures.
But the current fire devastation in California does suggest interventions, like better urban development, that could reduce the harm from fires in the near term.
“There is a lot we can do if we stop with this inordinate focus on climate change and focus on people,” Keeley said. “Global warming is very remote. Population growth and land planning — these are things we have control over.”